In photolithographic processes for fabricating semiconductor devices and the like, an exposure apparatus is employed to project, by way of a projection optical system, the image of a pattern on a photomask or reticle (collectively referred to hereinafter as a "reticle") onto a wafer, glass plate, or the like (collectively referred to hereinafter as a "wafer") which has been coated with photosensitive layer such as photoresist ("resist"). Semiconductor devices and the like have grown increasingly large in size and have been integrated to increasingly high densities in recent years. Accordingly, scanning-type exposure apparatus are being developed to handle the increased size of semiconductor devices.
As semiconductor devices are integrated to higher densities, increased resolving power is being demanded of projection optical systems. To satisfy this demand, it has become necessary to shorten the illumination wavelength and to increase the numerical aperture (NA) of the projection optical system.
However, shortening the illumination wavelength results in there being only a limited number of glass types available in practice due to absorption of light. Presently, the only glass types capable of being put to practical use at wavelengths of 300 nm and lower are synthetic quartz and fluorite. However, the Abbe numbers of these two materials are not sufficiently different to adequately correct chromatic aberration.
Because of the extremely high optical performance demanded, correction of the various aberrations must be such that practically no aberration remains. To accomplish this with a refractive projection optical system consisting only of lens groups (i.e., a dioptric system), many lens elements are required. Such optical systems have poor transmittance, and the need for many lens elements increases the cost of manufacturing such an optical system.
In contrast, all-reflective (i.e., catoptric) optical systems employing reflective surfaces such as concave mirrors and the like do not display chromatic aberration. Moreover, because the contribution to the Petzval sum of a mirror is opposite that of a lens, so-called catadioptric optical systems (i.e., systems which combine reflective and refractive elements) make it possible to practically eliminate not only chromatic aberration but also the other types of aberration as well, without increasing the number of lenses.
There have been a variety of proposals within the art for a catadioptric projection optical systems. For example, Japanese Examined Patent Application (Kokoku) No. H7[1995]-117648, Japanese Laid-Open Patent Application (Kokai) No. H6[1994]-300973, Japanese Laid-Open Patent Application (Kokai) No. H5[1993]-88089, Japanese Laid-Open Patent Application (Kokai) No. H3[1991]-282527, PCT/EP95/01719, and others disclose catadioptric reduction projection optical systems having an optical-path-changing beam splitter for inputting and outputting a light beam to and from a reflecting system.
In the aforementioned prior art, it is necessary to employ a beam splitter having a transmissoreflective (i.e., partially transmitting, partially reflecting) surface for splitting the optical path. It is necessary that this beam splitter be in the shape of a prism to prevent occurrence of asymmetrical aberrations. Moreover, with the aforementioned prior art, increasing the numerical aperture (NA) at the image side to increase resolving power requires a large prism that is 200 mm or longer on one side. Manufacture such a large prism is extremely difficult. In particular, nonuniformity and distortion (or strain) in the interior diminishes resolving power.
Japanese Laid-Open Patent Application (Kokai) No. H3[1991]-282527 and the other references cited above, may utilize a polarizing beam splitter. Such beam splitters require a multilayer transmissoreflective film on the transmissoreflective surface. The film is such that incident light is transmitted if it is P-polarized and reflected if it is S-polarized. This prevents the occurrence of luminous energy losses and stray light such as that due to flare or the like. A quarter-wave plate for changing polarization state is also necessary. However, as the polarizing beam splitter, quarter-wave plate, or other such optical element increases in size, its manufacture becomes extremely difficult. Also, the cost to manufacture such components is extremely high.